SiC-Si3 N4 Composite system for special heat-resisting ceramic materials and its fabrication method

ABSTRACT

A SiC-Si 3  N 4  composite system for special heat-resisting ceramic materials which is fabricated by firing in a nitriding gas atmosphere a green compact prepared from and composed of, as starting materials, silicon powder and an organic silicon polymer containing carbon and silicon atoms as the major skeletal components, whereby said composite system, as a final fired compact has an interwoven texture of SiC and Si 3  N 4  with sufficient micro gaps to absorb thermal stresses, wherein the quantitative ratio by weight of SiC to Si 3  N 4  in said composite system being in the range of 5%-20%:95%-80%.

BACKGROUND OF THE INVENTION

The present invention relates to a SiC-Si₃ N₄ composite system forspecial heat-resisting ceramic materials and its fabrication method. Themajor object of the present invention is to provide a SiC-Si₃ N₄composite system for special heat-resisting ceramic materials havingimproved resistances to thermal shock and fracture due to thermalfatigue and its fabrication method. The special heat-resisting ceramicmaterials of the present invention can be produced by firing in anitriding gas atmosphere a green compact composed of silicon powder andan organic silicon polymer containing silicon and carbon atoms as themajor skeletal components such final fired compact can have the noveltexture of interwoven SiC and Si₃ N₄ is therein fired, resulting inmarkedly improved physico-chemical properties such as high resistancesto thermal shock, to fracture due to thermal fatigue and to oxidation.

The organic silicon polymer referred to in the present invention hasbeen originally invented by Prof. Seiji Yajima et al, Institute ofMetallic Materials, Tohoku University as a family of organic siliconpolymers containing silicon and carbon atoms as the major skeletalcomponents. It is world-widely known that Prof. Yajima et al. haveinvented SiC fibers derived from such organic silicon polymers. Thereare now many relevant papers and patent specifications available on thisinvention. In addition, inspired by their invention, many relevantinventions have followed concerning the composition of SiC fibers withmetallic or non-metallic materials, or the utilization of the organicsilicon polymer as binding material for metallic or non-metallic powderand the employment of the organic silicon polymer as the startingmaterial for SiC fired compacts, thus opening a new field in theresearches of inorganic fibers, high-strength materials andheat-resisting materials.

SUMMARY OF THE INVENTION

A major object of the present invention is to provide a composite systemfor fabrication of special heat-resisting ceramic materials havingmarkedly improved physico-chemical properties such as high resistancesto thermal shock, to fracture due to thermal fatigue and to oxidation,employing above-mentioned silicon powder and the organic silicon polymeras starting materials. Basically, the organic silicon polymer employablein the present invention has the following unit structures (i) to (v):##STR1## wherein R₁ is --CH₃ ; R₂, R₃ and R₄ may be a member or amixture of at least two selected from the group consisting of hydrogen,alkyl, aryl, (CH₃)₂ CH--, (C₆ H₅)₂ SiH-- and (CH₃)₃ Si--. k, l, m and nare the mean number of repetitions of the unit structure in ( ) or { }and usually they are in the following ranges: k=1-80; 1=15-350, m=1-80;n=15-350. The mean molecular weight of the polymer is in the range of800-20000. In structure (iii), M is a metallic or non-metallic elementand, for example, Si, B, Ti, Fe, Al, or Zr. It may be derive from thestarting materials for preparation of the polymer having structure (iii)or from necessary catalysts. R₅, R₆, R₇ and R₈ may be a member or amixture of at least two members selected from the group consisting ofhydrogen, alkyl, aryl, (CH₃)₂ CH--, (C₆ H₅)₂ SiH-- and (CH₃)₃ Si--.Depending on the valency of M and the structure, any one or more of R₅,R₆, R₇ and R₈ may be absent.

(v) Compounds that contain any one or more of unit structures (i)-(iv)as partial unit structures in their chain or three-dimensionalconstruction; or the mixture of such compounds.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 explains the changes in the products synthesized from the organicsilicon polymer and silicon particles in the compact with increasingtemperature (temperature taken on abscissa; products taken on ordinate).The arrows define the range of synthesis of the relevant product. (a)shows the amorphous material of Si and C produced from the organicsilicon polymer by thermal denaturation; (b) β-SiC derived from (a); (c)β-SiC produced anew by reaction of Si particles with excess carbonderived from (a); and (d) Si₃ N₄ produced by reaction of N₂ gas withsilicon particles that are not involved in (c).

FIG. 2 reproduces the micrographs of the cracking surfaces taken with ascanning electron microscope. (A) is the material of the presentinvention in Example 4 and (B) reaction-sintered Si₃ N₄.

FIG. 3 presents a bidimensional model of the microstructure of (A) inFIG. 2. (1) is Si₃ N₄ ; (2) β-SiC; and (3) a pore.

FIG. 4 explains the difference in resistance to oxidation of thematerial of the present invention (A) in Example 5 from the conventionalproduct. The abscissa shows the period of time in hours and the ordinatethe weight increase due to oxidation (mg/cm²). (B) is hot-pressed Si₃ N₄with addition of Al₂ O₃ and Y₂ O₃ ; and (C) reaction-sintered Si₃ N₄.

DETAILED DESCRIPTION OF THE INVENTION

Prof. Yajima et al. have discovered that the heat treatment of theorganic silicon polymer in the non-oxidative atmosphere producesamorphous Si-C material which is further transformed to β-SiC containingfree carbon.

From the viewpoint of energy saving, various parts of heat engines havenow been reexamined for improved thermal efficiency. In this line ofapproach, the recent research activity has switched from metallicmaterials to heat-resisting ceramics which can withstand hightemperature. Among possible non-oxide compounds, nitrides (for example,Si₃ N₄, sialon and AlN) and carbides (for example, SiC) are particularlysought after since these materials are stable and have high strength andresistance to thermal shock at high temperature atmosphere. Furthermore,refractories or refractory material made of or containing such nitridesor carbides are also under extensive research activities.

In the course of research which has aimed at the mineralization of theorganic silicon polymer for the production of ceramics, the presentinventors have found that when SiC and Si₃ N₄ are produced by reactionin the above promising ceramics, such that they coexist in the sametexture, such ceramics could, among others, provide far betterresistances to thermal shock as well as to fracture due to thermalfatigue than those of the ceramics which contains either one of thecomponents. It is considered that the texture of the ceramic body playsa decisive role for providing resistances to thermal shock and tofracture due to thermal fatigue. Conventionally, Si₃ N₄ and SiCmaterials are fabricated by hot-press sintering,normal-pressure-sintering or reaction-sintering and these methodsprovide the ceramics physico-chemical properties specific to respectivemethods. As Si₃ N₄ and SiC have poor intrinsinc self-sinterability, itis very difficult to avoid the liquid-phase sintering of thesecond-phase-glassy dispersoid by fabrication methods other thanreaction-sintering. When the amount of the second-phase-glassydispersoid is large, it is very easy to sinter. However, the finalcomposite system suffers from the deteriorated physico-chemicalproperties as high temperatures, so that it cannot serve as specialheat-resisting material or high-temperature structural material. Forreduction of the second-phase glassy dispersoid, devitrifaction andsolid solution are utilized. For instance, at an initial stage of Si₃ N₄study, MgO was found to be useful as binder. However MgO has worked inits entirety as the glassy-dispersoid phase to bind Si₃ N₄ particles,simply providing high densification. Y₂ O₃ with or without Al₂ O₃ canproduce the devitrifaction phase, but does not lead to the completeabsence of the glassy-dispersoid phase. In other words, high-density Si₃N₄ thus obtained unavoidably suffers from sharp lowering of strength ata temperature above 800° C. to 1000° C.

High-temperature strength, environmental stability, thermal-stressresistance and anti-creeping property are essential properties forspecial heat-resisting materials and high-temperature structuralmaterials. As far as ceramics are concerned, resistance to thermalstresses is the most important property among them. In general, thermalstresses are classified into the drastically changing stress that isobserved on rapid heating and cooling and the repeated stress thatoccurs on temperature variation. In metallic materials which haveplasticity to some extent, plastic deformation serves to reduce suchstresses, whereas ceramics, particularly highly covalent materials, haveno such function (plastic deformation) as in metals having the metallicbond, because the type of chemical bondage is fundamentally different.Thus final fired products (ceramics) of high-density having thecontinuous bondage are very poor in resistance to thermal stresses.

The present inventors have found it possible to confer a bufferingfunction to such brittle (high prone to thermal stresses) ceramics bytexture control. Particularly we have succeeded in synthesizing in thesame texture both Si₃ N₄ and SiC which have excellent physico-chemicalproperties such as high-temperature strength, environmental stabilityand resistance to abrasion in spite of the low coefficient of thermalexpansion whereby the micro gaps produced between Si₃ N₄ and SiC is usedto absorb thermal stresses. More concretely speaking, a mixture ofsilicon powder and the organic silicon polymer as the starting materialis molded and then subjected to the heat treatment at a temperature from800° C. to 1800° C. in nitrogen gas atmosphere or nitrogen-ammonia gasmixture atmosphere. Due to their heat treatment, β-SiC is formed fromthe organic silicon polymer and also by the reaction of the siliconpowder with the released carbon. Furthermore N₂ reacts with Si to formα-Si₃ N₄ and β-Si₃ N₄. Microcrystals (less than tens of micron indiameter) of SiC, α-Si₃ N₄ and β-Si₃ N₄ are present in the form ofinterwoven texture without chemical bondage and/or without solidsolution among them, resulting in the micro gaps formation between SiCand Si₃ N₄. The final fired product thus obtained has interwoven textureof SiC and Si₃ N₄ with appropriate gaps therebetween.

Conventional SiC-Si₃ N₄ composite systems were, for example, fabricatedby firing a mixture of silicon powder with SiC powder or SiC fibers in anitrogen gas atmosphere at a temperature above 1200° C.(reaction-sintering); or sintering a mixture of Si₃ N₄ powder and SiCpowder or fibers with a Si₃ N₄ -sintering agent (for example, MgO, Al₂O₃ and Y₂ O₃) at a temperature of about 1600° C. (hot-press sintering);or baking a mixture of Si₃ N₄ and the organic silicon polymer in anon-oxidizing gas atmosphere at a temperature in the range of 1000° C.to 1800° C. (Japanese laid open publication 154816/1977 and Japaneselaid open publication 28309/1979).

The SiC-Si₃ N₄ composite system of the present invention fundamentallydiffers from traditional SiC-Si₃ N₄ composite materials in the fact thatSiC and Si₃ N₄ have interwoven texture with appropriate micro gaps toabsorb thermal stresses. In other words, traditional SiC-Si₃ N₄composite materials have no such interwoven texture in the final firedproducts, because the size and form of SiC and Si₃ N₄ in the startingmaterial determine the final structure and texture of the firedproducts.

The fabrication process of the present invention will be explained indetail hereinafter with the organic silicon polymer and silicon powderas the starting material.

When a green compact made of the mixture of the organic silicon polymerand silicon powder is fired in a nitriding gas atmosphere, the organicsilicon polymer primarily turns to a liquid of a low vicosity at arelatively low temperature (around 300° C.). At this stage, substituentsR₁ -R₈ in structures (i)-(v) of the organic silicon polymer (hydrogen,alkyl, aryl, (CH₃)₂ CH--, (C₆ H₅)₂ SiH--, (CH₃)₃ Si-- and the like)escape as volatile compounds, while the skeletal components of carbonand silicon become amorphous. β-SiC begins to form without crystallattice around 800° C. in the order of few to hundreds molecules fromamorphous Si and C. In other words, few to hundreds of β-SiC moleculesare scatteredly present in an amorphous mixture of Si and C, the mixturewith an excessive amount of C. At a temperature above 1000° C., theproduction of β-SiC is further accelerated by the reaction of siliconpowder with excess carbon. More particularly, the silicon particles inthe compact react with excess carbon that is previously produced fromthe organic silicon polymer on thermal denaturation, providing β-SiCwhich then chemically binds with the initially produced β-SiC in theamorphous material between the silicon particles. Above 1200° C., whileβ-SiC is produced by thermal denaturation of the organic silicon polymerand also by reaction of silicon particles with excess carbon, thesilicon powder particles that are not involved in the above reactionwith excess carbon start to react with N₂ gas to yield Si₃ N₄. Afterthese processes, the reaction temperature is raised above the meltingpoint of Si so that no silicon molecules remain unreacted withoutnitriding in the final fired product. It should be noted, however, thata high temperature above 1800° C. will cause the decomposition of Si₃N₄, giving the porous texture.

FIG. 1 summarizes the above-described processes of reaction. In FIG. 1,the transformation of products from the green compact made of themixture of the organic silicon polymer and silicon particles ispresented with increasing temperature of the reaction (reactiontemperature taken on abscissa; reaction products taken on ordinate). Thearrows defines the range of temperature in which each listed product wasproduced. The range was determined by X-ray diffraction. (a) is theamorphous material of silicon and carbon formed from the organic siliconpolymer by thermal denaturation; (b) is β-SiC derived from (a); (c) is anew β-SiC produced by reaction of the silicon particles in the compactwith excess carbon of (a); (d) is Si₃ N₄ obtained by reaction of N₂ gaswith the silicon particles that are not involved in (c).

In the reaction processes summarized above, the silicon particles areattacked from their surface by excess carbon derived from the organicsilicon polymer at reaction stage (c) of FIG. 1 and thus lose theiroriginal shape, forming the chemical bondage with (a) and (b) of FIG. 1.Namely, β-SiC of (a)-(c) and the SiC amorphous material tridimensionallypenetrate the space between silicon particles in the compact and formthe chemical bondage with them. At stage (d) the unreacted siliconparticles are nitrided to give Si₃ N₄ which constructs the interwoventexture with β-SiC without chemical linkage. This was experimentallyconfirmed.

The SiC-Si₃ N₄ composite material obtained by the above processescontains no such oxidative glassy-dispersoid phase as is observed in thehot-press or atmospheric-pressure-sintered Si₃ N₄ compacts which is amajor cause for the reduced strength at a high temperature. In addition,compared with traditional SiC-Si₃ N₄ composite materials that arelargely governed by the particle size, shape and mixing of startingmaterials, the special heat-resisting ceramics of the present inventionshow a markedly improved high-temperature strength and resistances tothermal shock and fracture due to thermal fatigue.

The advantages of the SiC-Si₃ N₄ composite system of the presentinvention will be elucidated in the following examples.

EXAMPLE 1

The mixed organic silicon polymer of unit structures (i) and (ii) and99.3% pure silicon powder (particle size <44μ) were mixed at mixingratios listed in Table 1 and then fired as described below. Atetrahydrofuran solution of the organic silicon polymer and the siliconpowder (particle size <44μ) were mixed for 5 hours in a hardenedstainless steel pot mill. After the tetrahydrofuran was evaporated off,the mixture was compacted to a rectangular plate of 20×20×80 mm at amolding pressure of 800 kg/cm². The compact was heated to 1500° C. inthe atmosphere of nitrogen gas at a heating speed of 100° C./hour andheld for 10 hours at the above temperature. The fired plate wasgradually cooled in the furnace and then subjected to physico-chemicalmeasurements. Table 1 shows the results of measurements and Table 2presents the quantitative ratios of the formed phases. The results inTable 2 were calculated from the calibration curve which was preparedwith crystobaryte as the standard material by X-ray diffraction.However, as the strongest peak of β-SiC (d=2.51 A) is least reproduciblebetween the peaks of α-Si₃ N₄ and β-Si₃ N₄, a peak of d=1.54 A wasemployed for measurement.

                  TABLE 1                                                         ______________________________________                                                        organic                                                              silicon  silicon  bulk   open   bending*                               specimen                                                                             powder   polymer  specific                                                                             porosity                                                                             strength                               numbers                                                                              wt %     wt %     gravity                                                                              %      kg/mm.sup.2                            ______________________________________                                        1      95        5       2.49   19.2   30.0                                   2      90       10       2.54   16.7   28.3                                   3      80       20       2.64   13.4   27.4                                   4      65       35       2.73   7.9    29.3                                   5      60       40       2.23   26.2   unmeasura-                                                                    ble**                                  ______________________________________                                         Note:                                                                         *The bending strength was measured at a span distance of 30 mm using a cu     specimen of 5 × 5 × 50 mm.                                        **As much volatile product occurred from the organic silicon polymer, the     fired plate became highly porous and deformed.                           

                  TABLE 2                                                         ______________________________________                                        spec-                                                                              silicon organic                                                          imen pow-    silicon                                                          num- der     polymer  β-SiC                                                                          β-Si.sub.3 N.sub.4                                                              α-Si.sub.3 N.sub.4                                                             Si                                  bers wt %    wt %     wt %  wt %   wt %   wt %                                ______________________________________                                        1    95       5       2.7   70.0   24.8   less                                                                          than 0.5                            2    90      10       5.1   63.2   30.1   less                                                                          than 0.5                            3    80      20       11.0  59.5   27.3   less                                                                          than 0.5                            4    65      35       20.5  46.7   29.8    1.2                                5    60      40       24.9  15.1   54.2    7.3                                ______________________________________                                    

With the above-defined organic silicon polymer, the upper limit of itscontent in the composition was 35 wt.%. In other words, the productionof β-SiC should be maintained below 20.5 wt.%. Otherwise, the finalfired product deformed.

EXAMPLE 2

Fired products Nos. 1-4 listed in Tables 1 and 2 were tested forresistance to thermal shock as described below. A rectangular specimenof 5×10×30 mm was cut and then held at 1200° C. for 20 minutes in thestream of nitrogen gas in a circular electric furnace. By tilting thefurnace, the hot specimen was abruptly cooled in running water. After 10minutes' cooling in running water, the plate was made dry and paintedwith various dyes for detection of a crack. The above thermal treatmentwas repeated until a crack was found in the specimen. Resistance tothermal shock was judged on the number of repetitions of the thermaltreatment necessary for occurrence of cracks. The results of test aresummarized in Table 3.

In contrast to the composite material of the present invention,traditional materials ((a) reaction-sintered Si₃ N₄ ; (b)reaction-sintered SiC; (c) reaction-sintered SiC with bound Si₃ N₄ ; (d)hot-pressed Si₃ N₄ with addition of MgO; (e) hot-pressed SiC withaddition of B₂ O₃) were similarly subjected to the thermal shock test.Table 4 shows the test results.

                  TABLE 3                                                         ______________________________________                                                       specimen numbers (product)                                                    of the present invention)                                                     1      2       3      4                                        ______________________________________                                        resistance to    9        28      33   39                                     thermal shock (number                                                         of repetitions of the thermal                                                 treatment)                                                                    ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                                  product of                                                                    the present                                                                   invention                                                                     (No. 3) a      b      c    d    e                                   ______________________________________                                        open porosity (%)                                                                         12.9      21.3   18.2 12.3 1.7  3.2                               bending strength                                                                          28.1      19.8   12.2 15.6 49.9 37.3                              (room temperature)                                                            (kg/mm.sup.2)                                                                 modulus of rupture                                                                        26.5      20.1   13.1 14.3 27.6 21.1                              (1400° C.)                                                             (kg/mm.sup.2)                                                                 resistance to                                                                             35        5      8    10   3    4                                 thermal shock                                                                 (number of                                                                    repetitions of the                                                            thermal treatment)                                                            ______________________________________                                    

These results indicate that the formation of β-SiC in the final firedproduct below 5% by weight (specimen No. 1) resulted in poor resistanceto thermal shock.

Thus the favorable effect of β-SiC in the final fired product starts toappear at 5 wt.% and ends at an upper limit of 20 wt.%, as is apparentfrom Example 1. The clear advantage of the product of the presentinvention over traditional Si₃ N₄, SiC and SiC-Si₃ N₄ compositematerials can be seen in about 3-fold increase in resistance to thermalshock.

EXAMPLE 3

The product of the present invention (specimen No. 3) and referencematerials (a)-(e) listed in Example 2 were tested for resistance tofracture due to thermal fatigue as described below. A rectangular plateof 5×5×50 mm was cut out of the final fired product and 1% of theflexural strength was applied as an initial load through the head (r=1mm) positioned in the middle of the 30 mm span. With the total flexurelimited to 10μ, the specimen plate was pulsated while the head followedthe resulting displacement. The above-described test conditions werereproduced in a furnace which was maintained at 1300° C. The specimenplate was pulsated at a speed of 30 pulsations/second. Table 5 shows thenumber of pulsations until the specimen plate was fractured, ornon-followability of the test head.

                                      TABLE 5                                     __________________________________________________________________________    product of the                                                                present invention                                                             (specimen No. 3)                                                                             a     b     c     d   e                                        __________________________________________________________________________    number of                                                                           more than                                                                              2-4 × 10.sup.4                                                                6-8 × 10.sup.5                                                                2-5 × 10.sup.5                                                                --  --                                       pulsations                                                                          4 × 10.sup.7                                                      necessary                                                                     for                                                                           fracture                                                                      note  unbreakable                                                                            broken                                                                              broken                                                                              broken                                                                              incomparable                                                                  because of                                                                    large                                                                         deformation                                  __________________________________________________________________________

Superiority of the material of the present invention to traditional Si₃N₄, SiC and SiC-Si₃ N₄ composite systems was clearly recognized inresistance to fracture due to thermal fatigue.

EXAMPLE 4

The material of the present invention (specimen No. 3) in Example 2 wasused to fabricate a tubular fired object (outer diameter 150 mm;thickness 5 mm; length 550 mm) by extrusion-molding. For comparison, thetubular fired objects of the same size were prepared with (a)reaction-sintered Si₃ N₄, (b) reaction-sintered SiC and (c)reaction-sintered SiC with bound Si₃ N₄. The inner surfaces of the tubeswere subjected to the test for resistance to repeated heating. Using anoxygen-propane burner placed at one end of the tubular burned object,the flame passing through the tube was appropriately adjusted so thatthe temperature of the flame reached a maximum in the middle of thetube. The maximum temperature was measured to be 1480°-1530° C. with anoptical pyrometer. After the flame was passed through the tube for 5minutes, the burner was removed and the test tubular fired object wasallowed to cool for 5 minutes. The above heat treatment was repeateduntil a crack was produced on the test surface and the number ofrepetitions of the heat treatment was recorded, Table 6 gives the testresults.

                  TABLE 6                                                         ______________________________________                                                  product of the                                                                present invention                                                             (specimen No. 3)                                                                         a       b      c                                         ______________________________________                                        number of                                                                     repetitions                                                                   of the heat 380          85      82   73                                      treatment                                                                     necessary for                                                                             more than    98      105  121                                     crack       410                                                               production                                                                    ______________________________________                                    

The excellent property of the product of the present invention is veryapparent from Table 6. The final fired object of the present inventionproduced no crack even after repeated heat treatments for three days,whereas the reference products gave clear cracks.

It is apparent from the above-described findings that in comparison withtraditional Si₃ N₄, SiC and SiC-Si₃ N₄ composite systems, the newmaterial of the present invention derived from the organic siliconpolymer and silicon powder has markedly improved properties inresistance to thermal shock and fracture due to thermal fatigue.

These improved properties of the product of the present invention can beattributed to the interwoven microstructure or complicated texture ofSi₃ N₄ (produced from silicon powder by nitriding) with β-SiC (producedfrom the organic silicon polymer by thermal denaturation; and producedby reaction of silicon particles with excess carbon derived from thesaid polymer).

FIG. 2 shows the micrographs of the cracking surfaces of the material ofthe present invention (A) and reaction-sintered Si₃ N₄ (B) taken with ascanning electron microscope. As is apparent from the microstructure ofthe material of the present invention, Si₃ N₄ crystals are surrounded byβ-SiC produced from the organic silicon polymer and by β-SiC produced byreaction of silicon particles with excess carbon derived from the saidpolymer, forming very complicated, interwoven patterns of β-SiC amongSi₃ N₄ crystals. In contrast, reaction-sintered Si₃ N₄ shows sharpcrystals of Si₃ N₄ without stuffing inbetween.

FIG. 3 is a bidimensional model of the microstructure of (A) consideredeasily from the micrograph in FIG. 2. In FIG. 3, the symbols have thefollowing meanings: β-SiC; Si₃ N₄ crystals or micro-agglomerates; □pores.

As β-SiC and Si₃ N₄ interweave each other without chemical binding onthe surface of their contact in the micro-crystal region, micro gaps arenecessarily formed which give a dimensional effect for absorption ofthermal stresses.

EXAMPLE 5

The material of the present invention was tested for resistance tooxidation as described below. The test was carried out at 1400° C. for200 hours in the stream of dry oxygen in a circular electric furnace. A5×5×20 mm sample plate was cut out and polished with 0.5μ diamond pasteparticles. The surface area was measured with a micrometer. Fordetermination of resistance to oxidation, the weight increase in theunit area (1 cm²) was measured initially five times in the first 60hours (every 12 hours) and then once every 24 hours. The sample platewas supported on a knife edge of compact alumina. The test results areshown in FIG. 4 where the ordinate represents the weight increase in theunit area (mg/cm²) and the abscissa the period of time in hours, A isthe material of the present invention; B being the hot-pressed Si₃ N₄with addition of Al₂ O₃ and Y₂ O₃ (open porosity 1.4%); and C being thereaction-sintered Si₃ N₄ (open porosity 22.7%).

In comparison with hot-pressed Si₃ N₄ with addition of Al₂ O₃ and Y₂ O₃,the material of the present invention showed about 2-fold higherresistance to oxidation.

As described above in details, the present invention provides theSiC-Si₃ N₄ composite system for special heat-resisting ceramics havingimproved resistances to thermal shock and to fracture due to thermalfatigue and high resistance to oxidation, which is produced by nitridinga molded object composed of the organic silicon polymer and siliconpowder as the strating materials. These improved properties can beattributed to the new microstructure of interwoven SiC and Si₃ N₄ in themicrocrystal region (less than tens of micron in diameter) containingappropriate micro gaps for absorption of thermal stresses, which hasnever been observed in conventional SiC-Si₃ N₄ composite materials.

As reaction-sintering is employed in the present invention, many moldingmethods such as extrusion-molding, injection-molding, casting-molding,isostatic-press-molding and die-press-molding can be utilized. Inaddition, as the final product can take any shape, the present inventionis widely applicable for special heat-resisting materials andhigh-temperature structural materials. Furthermore the nitridingprocedure may be divided into several steps for easier mechanical workssuch as thread cutting, which results in more complicated shapes of thefinal products.

The improved strength of the material of the present invention canadvantageously be utilized as substitutes for high-temperature metalparts in automobile engine parts, heat exchange pipes, turbine parts,radiant tubes jigs and supports. The excellent resistances of thematerial of the present invention to thermal shock and fracture due tothermal fatigue can effectively be utilized in various industrial fieldssuch as ceramic manufacture, atomic energy utilization, chemistry,industrial chemistry and steel-making industries.

What is claimed is:
 1. An SiC-Si₃ N₄ composite system for heat resistantmaterials, comprising crystals of β-SiC, α-Si₃ N₄ and β-Si₃ N₄, saidα-Si₃ N₄ and β-Si₃ N₄ crystals being surrounded by said β-SiC crystalsforming interwoven textures of β-SiC among said α-Si₃ N₄ and β-Si₃ N₄crystals without chemical bonding to provide micro gaps between saidβ-SiC and said α-Si₃ N₄ and β-Si₃ N₄ crystals for absorption of thermalstresses.
 2. The composite system of claim 1 wherein the weight ratio ofSiC:Si₃ N₄ is in the range of 5%-20%:95%-80%.
 3. The composite system ofclaim 2 which is fabricated by firing in a nitriding gas atmosphere, agreen compact composed of silicon powder and an organic silicon polymercontaining structural units selected from the group consisting of:##STR2## and mixtures thereof, with any one or more of structural units(i)-(iv) forming part of a larger polymeric chain,wherein R₁ is CH₃, R₂,R₃, and R₄ are selected from the group consisting of hydrogen, alkyl,aryl, (CH₃)₂ CH--, (C₆ H₅)₂ SiH--, (CH₃)₃ Si--, and mixtures thereof, k,l, m, and n represent mean number of repetitions of the respectivestructural units, and in structural unit (iii), M is either a metallicor non-metallic element, R₅, R₆, R₇ and R₈ are selected from the groupconsisting of hydrogen, alkyl, aryl, (CH₃)₂ CH--, (C₆ H₅)₂ SiH--, (CH₃)₃Si--, and mixtures thereof. and depending on the valency of M, any oneor more of R₅, R₆, R₇ and R₈ may be absent from the polymeric chain. 4.The composite system of claim 3 wherein k=1-80, 1=15-350, m=1-80,n=15-350, with the mean molecular weight of the polymer in the range ofabout 800-20,000.
 5. The composite system of claim 4 wherein M isselected from the group consisting of Si, B, Ti, Fe, Al, Zr, andmixtures thereof.
 6. The composite system of claim 4 wherein saidsilicon powder has a particle size equal to or less than about 44microns, and the mixing ratio by weight of silicon powder: organicsilicon polymer is in the range of 90%-65%:10%-35%.
 7. The compositesystem of claim 6 wherein the green compact is fired up to a temperaturein the range of 1200° C. to 1800° C.
 8. The composite system of claim 7wherein:(A) the green compact is initially fired to a temperature aboveabout 300° C. wherein the organic silicon polymer turns to a lowviscosity liquid, and substituents R₁ -R₈ of structural units (i)-(iv)escape as volatile compounds, with skeletal components of carbon andsilicon becoming amorphous; (B) the compact is fired to a temperatureabove about 800° C. wherein β-SiC crystals begin to form in theamorphous mixture of silicon and an excess of carbon; (C) the compact isfired to a temperature above about 1,000° C. wherein β-SiC crystalformation is accelerated by reaction of the silicon powder with anexcess of carbon, said β-SiC crystals formed in step (C) chemicallybinding with the β-SiC crystals produced in step (B) in the amorphousstructure between the silicon particles; (D) the compact is fired to atemperature above about 1200° C. wherein(1) β-SiC is produced by thermaldenaturation of the organic silicon polymer and by reaction of thesilicon powder with an excess carbon, and (2) silicon powder notinvolved in the reaction of (D) (1) starts to react with the nitridinggas to yield Si₃ N₄, and (E) reaction temperature is raised above themelting point of silicon up to about 1800° C. wherein no siliconparticles remain unreacted with nitriding gas in the final firedproduct, said Si₃ N₄ formed in steps (D) and (E) forming the interwoventexture with β-SiC without chemical linkage.
 9. A process forfabricating an SiC-Si₃ N₄ composite system for heat resistant materialshaving an interwoven texture of SiC and Si₃ N₄ with micro gaps betweenSiC and Si₃ N₄ crystals for absorption of thermal stresses, comprisingfiring in a nitriding gas atmosphere at a temperature of from 1200° C.to 1800° C., a green compact composed of 65% to 90% by weight of siliconpowder having individual particle size equal to or less than about 44microns and 35% to 10% by weight of an organic silicon polymer toproduce a final SiC-Si₃ N₄ composite system of 5% to 20% by weight ofSiC and 95% to 80% by weight of Si₃ N₄.
 10. The process of claim 9wherein said organic silicon polymer is of the structure selected fromthe group consisting of ##STR3## and mixtures thereof, with any one ormore of structural units (i)-(iv) forming part of a larger polymericchain,wherein R₁ is CH₃, R₂, R₃, and R₄ are selected from the groupconsisting of hydrogen, alkyl, aryl, (CH₃)₂ CH--, (C₆ H₅)₂ SiH--, (CH₃)₃Si--, and mixtures thereof, k, l, m, and n represent mean number ofrepetitions of the respective structural units, and in structural unit(iii), M is either a metallic or non-metallic element, R₅, R₆, R₇ and R₈are selected from the group consisting of hydrogen, alkyl, aryl, (CH₃)₂CH--, (C₆ H₅)₂ SiH--, (CH₃)₃ Si--, and mixtures thereof, and dependingon the valency of M, any one or more of R₅, R₆, R₇ and R₈ may be absentfrom the polymeric chain.
 11. The process of claim 10 comprising thesteps of(A) initially firing the green compact to a temperature aboveabout 300° C. wherein the organic silicon polymer turns to a lowviscosity liquid, substituents R₁ -R₈ of structural formulas (i)-(iv)escape as volatile compounds, and skeletal components of carbon andsilicon become amorphous; (B) firing the compact to a temperature aboveabout 800° C. wherein β-SiC crystals begin to form in the amorphousmixture of silicon and an excess of carbon; (C) firing the compact to atemperature above about 1,000° C. wherein β-SiC crystal formation isaccelerated by reaction of the silicon powder with an excess of carbon,said β-SiC crystals formed in step (C) chemically binding with the β-SiCcrystals produced in step (B) in the amorphous structure between thesilicon particles; (D) firing the compact to a temperature above about1200°0 C. wherein(1) β-SiC is produced by thermal denaturation of theorganic silicon polymer and by reaction of the silicon powder with anexcess of carbon, and (2) silicon powder not involved in the reaction of(D) (1) starts to react with the nitriding gas to yield Si₃ N₄, and (E)raising reaction temperature above the melting point of silicon up toabout 1800° C. wherein no silicon particles remain unreacted withnitriding gas in the final fired product, said Si₃ N₄ formed in steps(D) and (E) forming the interwoven texture with β-SiC without chemicallinkage.